Why is not feasible to operate bulbs a low voltages although it amount to high efficiency?

Lamps (not 'bulbs' ; we plant bulbs!) will only produce their rated power at their rated voltage. These values are printed on the glass envelope. Reducing the voltage applied to a lamp will, therefore, reduce the power it produces. In fact, there's not a simple, linear, relationship between a lamp's voltage and its power, and this means that even small reductions in voltage will result in large reductions in power.

Operating lamps, or any devices, come to than, at lower voltages really has nothing to do with efficiency. Certainly, operating a lamp below its rated voltage will most-certainly NOT result in high efficiency. Efficiency is defined as the ratio of output power to input power and has nothing to do with voltage.